1. Field of the Invention
The invention relates to a DC-DC power converter. More specifically, the invention relates to a converter in which the voltage across the main switch due to the leakage inductance of the transformer is clamped and a pair of capacitors play the role of lossless turn-off snubbers to recycle the leakage energy of the transformer instead of dissipating the energy so as to improve the circuit efficiency.
2. Description of the Related Art
A well-known conventional DC/DC flyback converter is shown in FIG. 1, where Lk 10 is the leakage inductance of the transformer T 12. The typical switching waveforms of FIG. 1 are shown in FIG. 2. When switch S 14 is turned off at t2, the leakage current charges the parasitic output capacitance of switch S 14 (output capacitance of S is not shown in FIG. 1), which causes a high voltage spike across switch S 14. After the leakage energy is completely released, the voltage across switch S 14 reaches its steady-state value. As a result, a high voltage rating for switch S 14 is required.
To eliminate this voltage spike, a number of circuit topologies have been reported in the literature. Among them, the R-C-D snubber, shown in FIG. 3 is one of the most popular ways to minimize the voltage spike and decrease dv/dt as shown in FIG. 2. The snubber circuit consists of diode D120, capacitor Cs 22 and resistor Rs 24. When switch S 14 is turned off, the leakage current in Lk 10 flows through diode D120 and charges capacitance Cs 22. The voltage across Cs 22 will change from zero to a clamped voltage so as to clamp the voltage across switch S 14. When switch S 14 turns on , the energy stored in Cs 22 is dissipated by the the snubber resistor Rs 24 through conduction of switch S 14. In other words, the leakage energy of the transformer is first charged to Cs 22 and then is dissipated by the resistor Rs 24. Therefore, the leakage energy is dissipated by the snubber which limits the circuit efficiency. This means the voltage clamp is achieved at the expense of low conversion efficiency.
The invention is a DC-DC converter in which the voltage across the main switch due to the leakage inductance of the transformer is clamped and a pair of capacitors play the role of lossless turn-off snubbers to recycle the leakage energy of the transformer instead of dissipating the energy so as to improve the circuit efficiency.
The DC-DC converter has a voltage source that is connected to a switch. A transformer first primary winding is in series with a first capacitor. This first winding and first capacitor are connected across the voltage source and switch. A transformer second primary winding is in series with a second capacitor. They are also connected across the voltage source and switch. The transformer first and second primary windings have first and second leakage inductances respectively.
A first diode has one terminal connected to terminals of the transformer first primary winding and the first capacitor. The first diode also has a second terminal connected to the terminals of the transformer second primary winding and the second capacitor. The first diode and first capacitor form a first snubber circuit. The first diode and second capacitor form a second snubber circuit.
The transformer first and second primary windings provide energy to a transformer secondary winding. In so doing, the voltage across the main switch due to the leakage inductance of the transformer is clamped and the leakage energy of the transformer is recovered by charging the first and second capacitors and the delivered output by the magnetizing inductance instead of being dissipated by the first and second snubber circuits. The first and second snubber circuits are lossless and therefore circuit efficiency is improved.
The advantage of the inventive DC-DC converter is that the voltage across the main switch due to the leakage inductance of the transformer is clamped. In addition, the leakage energy is full recovered and directly transferred to the load, instead of being dissipated as in the snubber circuits in the prior art. A pair of capacitors play the role of lossless turn-off snubbers to recycle the leakage energy of the transformer instead of dissipating the energy so as to improve the circuit efficiency. As a result, the invented circuit has potential high power conversion efficiency and low cost.
Another objective of the invention is to use as few components as possible and use only one active switch to reduce the cost.